Ferro-electric random-access memories (F-RAM) or memory devices typically include a grid or an array of storage elements or cells, each including at least one ferro-electric capacitor or ferro-capacitor and one or more associated transistors to select the cell and control reading or writing thereto. The ferro-capacitor includes a ferro-electric material, such as Lead Zirconate Titanate (PZT), having a crystal structure with a dipole having two equal and stable polarization states. When an external electric field is applied across the ferro-capacitor, dipoles in the ferro-electric material will align or polarize in the field direction. After the electric field is removed, the dipoles retain their polarization state. This polarization state is read by applying a voltage across the ferro-capacitor through a plateline and a released bit-line initially pre-charged to 0V. The amount of charge generated depends on whether the electric field produced by the applied voltage causes the polarization state of the ferro-electric material to switch. For example, the response of the ferro-capacitor when the polarization is not switched, referred to as the unswitched or U-term, is linear or proportional to the applied voltage, and translates to data ‘0’ when a dataline is connected to the non-inverting side of a sense-amplifier and compared to a reference voltage. The response of the ferro-capacitor when the polarization is switched, referred to as the polarization switching or P-term, is non-linear, typically two times or more greater than the U-term, and translates to data ‘1’ when the dataline is connected to the non-inverting side of a sense-amplifier and compared to the reference voltage.
One problem with conventional F-RAM devices is that precise values for the P-term and U-term can vary for each cell in the array due process variations in the manufacture of the device. Thus, existing F-RAM design which uses a global reference voltage is programmed and placed in between the weakest U-term (one having the highest charge), and the weakest P-term (one having the lowest charge) of any F-RAM cell in the device. Furthermore, these values for the weakest U-term and P-term can vary over the lifetime of the device due to changes in the temperature, voltage and a number of read and/or write cycles the device has experienced. Thus, an effective F-RAM signal margin of conventional F-RAM devices is generally low, and decreases with use, leading to problems with reliability and a reduced operating life.
Accordingly, there is a need for an improved memory device using F-RAM cells and methods for operating the same to maximize signal margin and extend the operating life of the device.